The present disclosure relates to a sheet feed apparatus for feeding sheets such as paper sheets, and to an image forming apparatus.
Sheet feed apparatuses are broadly adopted in image forming apparatuses such as copiers, facsimiles, scanners, multifunction peripherals, and the like. For example, a sheet feed apparatus is used to automatically feed sheets such as paper sheets one by one, on which the image of the document sheet is transferred and each sheet is conveyed to an image forming apparatus, thereby allowing the image forming apparatus to print each image continuously.
To feed, one by one, a plurality of sheets placed on the tray as a bundle, the sheet feed apparatus causes the sheet bundle to abut a pick-up roller. The thickness of the sheet bundle varies depending on the number of sheets placed on the tray. As a result, it is necessary to cause the sheet bundle to abut the pick-up roller always appropriately. Therefore, in a typical sheet feed apparatus, the distance between the pick-up roller and the sheet mounting surface is adjusted based on the thickness of the sheet bundle. For this adjustment, a lifting/lowering plate (lift plate) may be used, wherein the lifting/lowering plate is attached to the sheet mounting surface in such a manner that the sheet mounting surface can be lifted and lowered. This typical sheet feed apparatus causes the upper surface of the sheet bundle to abut the pick-up roller by causing the lifting/lowering plate to push up the sheet bundle on the lifting/lowering plate. When the pick-up roller abuts a sheet, the pick-up roller feeds the sheet.
There are known various techniques for realizing stable sheet feeding in such a sheet feed apparatus. For example, there is known a configuration in which a torsion spring is adopted as a lift member for lifting the lifting/lowering plate. According to this configuration, the force that pushes up the lift plate is made variable by adjusting the elastic deformation of the torsion spring in correspondence with the paper sheet remaining amount. According to this configuration, an approximately uniform paper feeding pressure (a force that presses the upper surface of the sheet bundle against the pick-up roller) is generated in correspondence with the variation in the weight of the mounted paper sheets.
Furthermore, there is known a configuration in which a plurality of elastic members are used as the lift member for lifting the lifting/lowering plate. According to this configuration, the force that pushes up the lift plate is made variable by using an elastic member that generates a linear elastic force, and an elastic force that generates a non-linear elastic force.